iotests: Add test for change-related QMP commands

Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/118.out

+...........................................................
+----------------------------------------------------------------------
+Ran 59 tests
+
+OK

tests/qemu-iotests/group

@@ -122,6 +122,7 @@
 114 rw auto quick
 115 rw auto
 116 rw auto quick
+118 rw auto
 119 rw auto quick
 120 rw auto quick
 121 rw auto